Electronic devices typically require a connected (wired) power source to operate, for example, battery power or a wired connection to a direct current (“DC”) or alternating current (“AC”) power source. Similarly, rechargeable battery-powered electronic devices are typically charged using a wired power-supply that connects the electronic device to a DC or AC power source. The limitation of these devices is the need to directly connect the device to a power source using wires.
Wireless power transfer (WPT) systems typically use time-varying magnetic fields and the principle of magnetic induction or magnetic resonant induction to transfer power wirelessly. In accordance with Faraday's Law, a time-varying current applied to a transmitter coil produces a magnetic field that will induce a voltage in a receiver coil that is in close proximity to the transmitter coil. The induced voltage in the receiver coil is typically rectified and filtered to produce a substantially direct current (DC) voltage that can provide power to an electronic device or a rechargeable battery. Such wireless power transfer systems may use magnetic induction or magnetic resonant induction techniques, both of which emit magnetic flux in the “near-field.” Such near-field techniques are capable of transferring power only when the transmitter coil and the receiver coil are within a short distance from one another, typically on the order of a few centimeters or less.
The Wireless Power Consortium (WPC) was established in 2008 to develop the Qi inductive power standard for charging and powering electronic devices. Powermat is another well-known standard for WPT developed by the Power Matters Alliance (PMA). The Qi and Powermat near-field standards operate in the frequency band of 100-400 kHz. The problem with near-field WPT technology is that typically only 5 Watts of power can be transferred over the short distance of 2 to 5 millimeters between a power source and an electronic device, though there are ongoing efforts to increase the power. For example, some concurrently developing standards achieve this by operating at much higher frequencies, such as 6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methods instead of magnetic induction, they are based on the same underlying physics of magnetic induction. There also have been some market consolidation efforts to unite into larger organizations, such as the AirFuel Alliance consisting of PMA and the Rezence standard from the Alliance For Wireless Power (A4WP), but the technical aspects have remained largely unchanged.
FIG. 1 is a diagram of a prior art embodiment of a single coil structure for wireless power transfer. A transmitter 100 includes a power supply 110, a half-bridge 112, a capacitor 114, and a coil 116. Coil 116 is typically a flat spiral coil with a predetermined number of turns. Half-bridge 112 is controlled by a control circuit (not shown) to provide an alternating current to capacitor 114 and coil 116. The current is typically in the range of 100 KHz to 400 kHz. The capacitance value of capacitor 114 and the inductance value of coil 116 determine a resonant frequency for transmitter 100. The alternating current passing through coil 116 generates magnetic flux that can induce a current in a receiver coil (not shown).
One drawback of single coil wireless power transmitters is that the size of the single coil limits the size of the transmitting surface of the power transmitter. In single coil wireless power transmitters, the area of the single transmitter coil is limited by the magnetic field necessary to induce a sufficiently large current in a receiver coil. This limitation results from the fact that the magnetic flux produced by a coil is inversely proportional to its area. A small coil in the power transmitter makes its alignment with the receiver coil in the device to be charged more critical.
Current attempts to create larger wireless transmitter surfaces are problematic for a variety of reasons. One attempt involves simply enlarging the coil. But merely enlarging the area of a spiral coil causes the magnetic flux generated by the coil to be weaker, particularly in the middle of the coil. Another attempt is to use multiple coils, connected in series or in parallel, which would theoretically allow more than one coil to be engaged simultaneously in the wireless power transfer process. However, a wireless transmitter that includes multiple coils comes with its own set of drawbacks.
For example, multiple coils may also be connected together in parallel but if all coils are activated simultaneously, small differences in the coils' characteristics could cause unforeseen circulating currents and electromagnetic interference loops. Multiple coils may also be connected together in series. Series-connected coils may be switched in and out of the power transfer process as desired depending on the needs of the receiving device(s). However, the problem with creating configurable series-connected coils in this manner is that switching coils in and out of the wireless power transfer process changes the net series inductance of the circuit, which ultimately changes the resonant frequency of the transmitter. Changes in the resonant frequency could interfere with wireless power transfer and, in particular, adhering to a wireless power transfer standards (e.g., the Qi standard requires a resonant frequency of 100 kHz).
There is, therefore, an unmet demand for efficient wireless transmitters having transmitting surfaces of a customizable size, while maintaining the resonant frequency of the transmitter as the size of the transmitting surface changes.